The domain servers store information as a series of resource records (RRs), each of which contains a particular piece of information about a given domain name (which is usually, but not always, a host). The simplest way to think of a RR is as a typed pair of datum, a domain name matched with relevant data, and stored with some additional type information to help systems determine when the RR is relevant. For the purposes of message routing, the system stores RRs known as MX RRs. Each MX matches a domain name with two pieces of data, a preference value (an unsigned 16-bit integer), and the name of a host. The preference number is used to indicate in what order the mailer should attempt deliver to the MX hosts, with the lowest numbered MX being the one to try first. Multiple MXs with the same preference are permitted and have the same priority.
In addition to mail information, the servers store certain other types of RR's which mailers may encounter or choose to use. These are: the canonical name (CNAME) RR, which simply states that the domain name queried for is actually an alias for another domain name, which is the proper, or canonical, name; and the Well Known Service (WKS) RR, which stores information about network services (such as SMTP) a given domain name supports.